1. Field of the Invention
The present invention relates to digital sine wave generation, and, in particular, to frequency modulation for a digital sine wave generator with continuous phase and constant amplitude.
2. Description of the Related Art
Many communication applications require generation of a digital sine wave for which the frequency can be arbitrarily changed at any time, while maintaining i) constant amplitude, ii) continuous phase, and iii) the same sampling frequency. The digital sine wave generator output is commonly employed for data modulation. One common application employing sine wave generators is Frequency Shift Keyed (FSK) modulation used for caller ID in telephony, and other common applications include FM, MSK, and GMSK modulation. Ordinary second order digital resonators incorporating a digital sine wave generator do not maintain constant amplitude, continuous phase, and the same sampling frequency of the output signal from the digital sine wave generator. In such a resonator, changing a parameter controlling the output signal frequency of the digital sine wave generator causes the output signal amplitude to change. The amplitude change can be very large, and is dependent upon the phase of the output signal at the time the change occurs.
In order to overcome these shortcomings of the resonator, digital sine wave generators currently exhibit one or more limitations. Modulation of the sampling frequency requires a given implementation to allow for variable frequency dividers or frequency synthesizers that add significant complexity to the implementation's design. Sine wave lookup tables also require sizeable memory storage to achieve accuracy. Large sine wave lookup tables also add computational overhead to calculate i) the memory address stepping increments used to produce a sine wave at the desired frequency and ii) the starting address to satisfy phase continuity requirements. Finally, switching between multiple resonators requires an implementation to support as many resonators as necessary to accommodate the anticipated frequencies used. Switching between multiple resonators requires some technique to accomplish switching without phase discontinuity, since independent resonators operating at different frequencies are not continually phase aligned.